Process for catalyzing reduction in a Kraft recovery boiler

Abstract

A Kraft pulping process by which spent pulping liquor is burned in a recovery furnace 12 to reduce inactive alkali sulfates therein to active alkali sulfides while producing molten smelt that when added to water forms green liquor and residual dregs bearing catalytic iron. The dregs are separated from the green liquor and divided into first and second portions. The first portion of the dregs is discarded while the second portion of dregs required to maintain a concentration of iron in the smelt at from 0.05% to 5% is recirculated back to the furnace to provide optimum catalytic reduction of alkali sulfates to alkali sulfides. A quantity of alkali metal compounds is added at 24 to the pulping liquor in line 10 to serve as a temperature depressor that, together with the co-addition of said dregs, lowers the temperature of the molten smelt in furnace 12 from approximately 800° C. to a more favorable temperature of about 650° C. where maximum reduction of alkali sulfates to alkali sulfides will occur.

I claim: 
     
       1. A Kraft pulping process in which spent pulping liquor bearing inactive alkali sulfates and carbon is subjected to combustion in a recovery furnace to produce inorganic ash and to reduce the alkali sulfates to alkali sulfides, said process comprising the steps of introducing spent pulping liquor to the furnace, burning the spent pulping liquor in the furnace to form a molten smelt, removing the smelt from the furnace and adding said smelt to a quantity of water to produce green liquor containing iron-rich dregs, removing essentially all said dregs from the green liquor to provide a quantity of iron-rich dregs and a quantity of essentially iron-free green liquor, recirculating the essentially iron-free green liquor to a causticizer, dividing the iron-rich dregs into first and second portions, discarding only the first portion of the iron-rich dregs after having removed the valuable constituents therefrom, and directly recycling the second portion of the iron-rich dregs to the recovery furnace by introducing only the second portion of the iron-rich dregs into the spent pulping liquor immediately before introducing the spent pulping liquor into the recovery furnace thereby providing sufficient iron to catalytically reduce the alkali surface therein to alkali sulfide. 
     
     
       2. A Kraft pulping process as defined in claim 1 wherein the second portion of the iron-rich dregs includes sufficient iron that when added to the spent pulping liquor maintains an iron concentration in the smelt formed in the recovery furnace at from 0.05% to 5%. 
     
     
       3. A Kraft pulping process as defined in claim 2, including the addition of a melting point depressor to the spent pulping liquor before the spent pulping liquor is introduced into the recovery furnace to lower the temperature of the smelt formed therein thereby obtaining a maximum reduction of sodium sulfate to sodium sulfide. 
     
     
       4. A Kraft pulping process as defined in claim 3 wherein the melting point depressor is comprised of alkali metal compounds. 
     
     
       5. A Kraft pulping process as defined in claim 4 wherein the melting point depressor of alkali metal compounds and the co-addition of the iron-rich green liquor dregs lowers the temperature of the molten smelt in the recovery furnace to approximately 650° C. and optimizes the catalytic activity of the iron in reducing the sodium sulfate in the spent pulping liquor to sodium sulfide.